<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        div {
            width: 200px;
            height: 200px;
            background-color: pink;
        }
        .big{
            width: 500px;
            height: 500px;
            background-color: green;
        }
    </style>
</head>

<body>
    <div id="one"></div>
</body>
<script>
    var one = document.getElementById('one')
    // 驼峰命名法
    // 改变元素的样式
    // one.style.backgroundColor = 'red'
    // one.style.width = '500px'

    one.className = 'big'
</script>
</html>